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HomeCyber SecurityInfluencing Forwarding Habits with Coverage Based mostly Routing

Influencing Forwarding Habits with Coverage Based mostly Routing

It had been a scorching minute since I final put collectively a weblog, and I used to be fascinated about what is perhaps an fascinating matter. Properly, as is typical, I considered what I’d lately run throughout, or labored on, in my “day job” as a part of the engineering staff that builds and helps the lab environments for all of the Studying at Cisco coaching supplies.
On this specific day, I used to be reviewing the present configurations of the core community routers (layer 3 switches actually) in our knowledge facilities. I’m pretty new to this a part of the staff, and I used to be to find that we have been leveraging Coverage Based mostly Routing to govern the forwarding habits for several types of site visitors. I’m certain lots of you studying this weblog are conversant in the truth that there are at all times a number of methods to perform a process in networking (life actually, however undoubtedly in networking). As such, policy-based routing is a device within the community engineer’s toolkit that may usually be leveraged to deal with “odd enterprise necessities.”
And with that, I had a subject to make use of for this weblog and an accompanying video to kick off a brief video sequence referred to as “Technically Talking… with Hank Preston” on the Cisco U. by Studying and Certifications YouTube channel. Particularly, we’re going to have a look at the way to configure policy-based routing to affect forwarding habits. The why I’ll go away for an additional publish. 🙂
Additionally, for anybody finding out for the CCNP Enterprise certification, policy-based routing is on the ENARSI – Implementing Cisco Enterprise Superior Routing and Providers blueprint – “1.6 Configure and confirm policy-based routing.” 300-410 ENARSI is a focus examination that earns you the Cisco Licensed Specialist – Enterprise Superior Infrastructure Implementation certification.  So, it’s undoubtedly an important matter for the Cisco Studying weblog. Let’s dive proper in!
Setting the Stage
Earlier than we have a look at altering the standard routing and forwarding habits, let’s begin with the essential forwarding habits. For this exploration, I put the under community collectively in a Cisco Modeling Labs simulation. (You’ll find the topology file right here.)
The community topology used on this exploration of coverage based mostly routing and forwarding habits.
This community has two small LANs separated by a fundamental, single space OSPF community within the center. The prices within the OSPF community have been configured to make the perfect path from R1 to R5 by way of R3. We will see that in a pair methods.
First, let’s have a look at the interface prices on R1.
R1#present ip ospf interface transient

Interface PID Space IP Tackle/Masks Price State Nbrs F/C
Gi0/1.200 1 0 1 DR 0/0
Gi0/1.100 1 0 1 DR 0/0
Gi0/4 1 0 110 DR 1/1
Gi0/3 1 0 1 DR 1/1
Gi0/2 1 0 100 DR 1/1

Discover the prices for interface G0/2 and G0/4 (in the direction of R2 and R4) have a price of 100 and 110 respectively, whereas the price of G0/3 (in the direction of R3) is only one.
And now, we’ll confirm the routing desk entry for host H3 on R1.
R1#present ip route

Routing entry for
Identified by way of “ospf 1”, distance 110, metric 3, sort intra space
Final replace from on GigabitEthernet0/3, 00:23:02 in the past
Routing Descriptor Blocks:
*, from, 00:23:02 in the past, by way of GigabitEthernet0/3
Route metric is 3, site visitors share depend is 1

The routing desk lists the route as in the direction of R3 out interface G0/3 — precisely as we’d count on.
The ultimate examine shall be a hint route from host H1.
H1:~$ traceroute -n

traceroute to (, 30 hops max, 46 byte packets
1 5.534 ms 5.004 ms 3.038 ms
2 5.528 ms 5.531 ms 4.137 ms <- R3’s G0/1 interface
3 5.533 ms 5.656 ms 6.339 ms
4 14.180 ms 9.787 ms 7.908 ms

And no massive shocker right here, the second hop within the hint is certainly R3.
Let’s shake issues up a bit bit.
Suppose there was some cause that you simply needed to direct site visitors obtained at router R1 from host H1 destined for H3 to go by way of R2 . Possibly there was some site visitors evaluation that occurred on that router. Or maybe that hyperlink is extra dependable, even when slower. There are any variety of causes this may come up in a community design. The important thing half is that you simply don’t need to change ALL forwarding habits, simply a few of it. You may have a “coverage,” so to talk, that identifies some site visitors you need to modify. That is the place coverage based mostly routing, sometimes called PBR, is available in.
Coverage based mostly routing can appear sophisticated. To be truthful, if overused, it could make networks very sophisticated and arduous to take care of. Nonetheless, the technical fundamentals of PBR are fairly simple.
First, you want a technique to determine the site visitors that you simply need to apply the coverage to. Like many “matching” use instances in networking, that is usually performed with an access-list. So, right here’s the entry record that I’ll use to match the site visitors I’m enthusiastic about.
ip access-list prolonged H1-to-H3
10 allow ip host host

This single line prolonged ACL is all that’s wanted. I’m matching all IP site visitors from H1 to H3, however I might be extra particular, to a particular port as properly. Possibly simply net site visitors (tcp/80 & tcp/443) for instance.
Subsequent, a route-map is used to explain the coverage that we need to configure. The “coverage” is made up of “match” situations to determine the site visitors and “set” situations to make the “coverage based mostly adjustments” to the site visitors that was matched.
Right here is the route-map for my coverage instance.
route-map POLICY-BASED-ROUTING allow 10
description Visitors from H1 -> H3 route by way of R2
match ip tackle H1-to-H3
set ip next-hop

I’ve used the access-list I created in my “match ip tackle” command. And, I’ve indicated that when site visitors “matches” this coverage, I need to “set” the next-hop to be
And see the primary line within the configuration instance. It ends with the quantity “10.” This quantity identifies the place within the route map that this specific coverage entry holds.  A route-map might be made up of many coverage units – every with a “match” and “set” assertion.  On this manner, community engineers can have very granular management over how site visitors is forwarded within the community.  Fairly helpful proper!
Earlier than I am going a lot farther it’s undoubtedly vital to notice that route-maps are used for extra than simply coverage based mostly routing.  The route-map assemble can also be used as a part of high quality of service (QoS) configurations, routing protocol filtering, and BGP path manipulations.  So for those who discover the configuration choices accessible for match and set you’ll discover a number of different choices.  Most of those are used to be used instances aside from coverage based mostly routing.
The final step to finish the configuration of my coverage is to use it to the router interface. Since this coverage is about controlling site visitors from the LAN linked to interface Gig0/1 on R1, that’s the place I’ll apply it.
interface Gig0/1.100
ip coverage route-map POLICY-BASED-ROUTING

That’s it, we’ve configured coverage based mostly routing. Let’s take a look at to see if it’s working.

We’ll begin by rerunning the identical hint route command as earlier than and evaluating the outcomes.
1:~$ traceroute -n

traceroute to (, 30 hops max, 46 byte packets
1 7.306 ms 3.017 ms 3.337 ms
2 3.844 ms 4.335 ms 3.688 ms <- R2’s G0/1 interface
3 7.906 ms 5.125 ms 5.962 ms
4 8.951 ms 8.912 ms 7.348 ms

Take a look at that, site visitors is certainly going by way of R2 now. However let’s confirm that it’s only for site visitors to H3 by hint routing the site visitors to H4.
H1:~$ traceroute -n

traceroute to (, 30 hops max, 46 byte packets
1 3.681 ms 3.153 ms 2.563 ms
2 3.613 ms 3.185 ms 3.747 ms <- R3’s G0/1 interface
3 5.957 ms 7.555 ms 5.040 ms
4 14.915 ms 7.157 ms 7.853 ms

Yep, site visitors from H1 to H4 is certainly nonetheless following the “customary path” by way of R3. However what about site visitors from H2 -> H3?  Will or not it’s redirected by way of R2?
H2:~$ traceroute -n

traceroute to (, 30 hops max, 46 byte packets
1 7.284 ms 2.840 ms 3.173 ms
2 3.526 ms 4.514 ms 3.498 ms <- R3’s G0/1 interface
3 6.375 ms 7.212 ms 4.900 ms
4 6.642 ms 6.270 ms 5.884 ms

Nope, solely site visitors from H1 -> H3 is being redirected.
If we have a look at the routing desk on R1, we’ll see nothing has modified.
R1#present ip route

Routing entry for
Identified by way of “ospf 1”, distance 110, metric 3, sort intra space
Final replace from on GigabitEthernet0/3, 00:23:02 in the past
Routing Descriptor Blocks:
*, from, 00:23:02 in the past, by way of GigabitEthernet0/3
Route metric is 3, site visitors share depend is 1

There are a number of helpful instructions on the router to examine the standing of coverage based mostly routing.
First up, a fundamental “present” command value noting.
R1#present route-map

route-map POLICY-BASED-ROUTING, allow, sequence 10
Match clauses:
ip tackle (access-lists): H1-to-H3
Set clauses:
ip next-hop
Coverage routing matches: 12 packets, 756 bytes

This command offers “coverage match” statistics. We will see that once I ran this command there have been 12 matches to this point.
One other command that’s helpful is the “debug ip coverage” command. It offers helpful particulars in regards to the processing of the coverage as site visitors flows by way of the router. However as with all “debug” command, watch out utilizing it on a manufacturing system as it could put a heavy load on community gadgets if there may be a number of site visitors flowing by way of.
I’ll activate the debugging after which ship a single ICMP (ping) packet from H1 -> H3.
R1#debug ip coverage
Coverage routing debugging is on

*Apr 26 00:29:58.282: IP: s= (GigabitEthernet0/1.100), d=, len 84, FIB coverage match
*Apr 26 00:29:58.282: IP: s= (GigabitEthernet0/1.100), d=, len 84, PBR Counted
*Apr 26 00:29:58.282: IP: s= (GigabitEthernet0/1.100), d=, g=, len 84, FIB coverage routed

Evaluate the above output to the debug output once I ping H1 -> H4.
*Apr 26 00:31:00.294: IP: s= (GigabitEthernet0/1.100), d=, len 84, FIB coverage rejected(no match) – regular forwarding

Within the first instance, “FIB coverage match” signifies that the PRB coverage was triggered. And a following debug line exhibits that the site visitors was “FIB coverage routed.” That’s the PBR in motion. Evaluate that to the output from the second ping that’s “FIB coverage rejected (no match) – regular forwarding.” That output is fairly descriptive.

And with that, we’ve come to the top of this publish. I hope this brief have a look at coverage based mostly routing helped break it down and introduce you to a brand new know-how device which you could put into your toolkit. Possibly it’ll assist you to clear up a enterprise problem sometime. Or possibly it’ll assist you to in your preparation for the ENARSI examination or different research. Both manner, thanks for hanging out with me right this moment.

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